Hitherto, a powdered paint of an olefin polymer base, particularly, that of a polyethylene or its copolymer has been widely used for providing corrosion resistance, chemical resistance and stain resistance to metallic articles. For example, it is used for various articles such as draining boards, vegetable baskets, baskets for bicycles, shelves of refrigerators and the like, which can be often seen in our everyday life.
In the field of daily necessaries as described above wherein heat resistance is not particularly required, coating with a polyethylene or its copolymer is sufficient for attaining the above purpose. However, in the case that coated metallic articles are those to be used under high temperature conditions such as hot water pipes, peripheral parts of automobile engines and the like, sufficient resistance can hardly be obtained by coating with a polyethylene or its copolymer.
On the other hand, polypropylene has been widely used as a material for producing various molded articles because of its excellent heat resistance and dimensional stability as well as its small coefficient of thermal expansion. However, there are few cases that polypropylene is used in powder coating because, when a metal is subjected to powder coating with polypropylene, there are problems such as low adhesiveness between polypropylene and a metal and inferior elongation of a coating film obtained after coating.
Japanese Patent Kokai No. 59-30841 discloses a composition for fluidized bed dip coating to improve adhesion to a metal. In Example 6 thereof, there is disclosed that polypropylene is dryblended with polypropylene modified by treatment with maleic anhydride to obtain a composition, and a metal is subjected to fluidized bed dip coating by using this composition. The resulting coated material shows improved adhesiveness. However, adhesiveness is still insufficient, and low elongation of the coating film is scarcely improved.
Further, Japanese Patent Kokai Nos. 62-190265 and 62-190266 disclose improvement of adhesiveness in the case that a metallic article to be subjected to powder coating is used in a hot atmosphere as well as improvement of inferior elongation of a coating film of a polypropylene. The former application discloses a resin composition for powder coating which comprises a modified polypropylene obtained by graft copolymerization of a part or all of a crystalline random copolymer composed of propylene and .alpha.-olefin with an unsaturated carboxylic acid or its anhydride, a polyethylene, a metal oxide or hydroxide and a nucleating agent. From the viewpoint of the resin ingredients, the composition disclosed in the latter application is the same as that of the former application except that a vinylcycloalkane polymer having 6 or more carbon atoms is further added thereto. When a metal is coated with the compositions disclosed in both applications, adhesion and elongation of the resulting coating film are improved. However, adhesiveness and elongation in a hot atmosphere or in hot water dipping should be further improved.
In the case of a metal subjected to powder coating, good elongation of a coating film is one of the important requisites. For example, if elongation of a coating film is inferior, bared metal surface is exposed at an edge of a corner of a metallic article. Further, when the shape of a metallic article is complicated, the coating film is broken even by a small amount of stress or impact, which spoils the original purpose of the article. Still another problem is that cracking is liable to form by thermal stress.